Tides 1 CPF09 v1.3 Copyright © BSAC 2010 Chartwork and Position Fixing Tides

Chartwork and Position Fixing Tides 2 CPF09 v1.3 Copyright © BSAC 2010 Outline Tidal terms Depth and tides Tide tables ●times and heights of HW/LW ●springs and neaps ●depth of water for launch, recovery, diving and navigation Tidal diamonds & atlases ●slack time and duration ●rates for navigation Tidal hazards

Chartwork and Position Fixing Tides 3 CPF09 v1.3 Copyright © BSAC 2010 Tidal terms High water (HW) Low water (LW) Tidal range Flood Ebb Tidal cycle ~12.5 hours time State = position in tidal cycle

Chartwork and Position Fixing Tides 4 CPF09 v1.3 Copyright © BSAC 2010 Depth and tides Spring tides !greatest tidal range ○lower LW ○higher HW Neap tides !least tidal range ○higher LW ○lower HW Abbreviations ●MHWS MLWS MHWN MLWN HW LW HW LW New moon Full moon Quarter moons

Chartwork and Position Fixing Tides 5 CPF09 v1.3 Copyright © BSAC 2010 Tide tables TU 19 W 20 TH 21 F 22 SA SU 24 MO 26 W 27 TH 28 TH 29 SA SU Time m TU 17 MO Tides are predictable ●calculated for standard ports ●differences for secondary ports ●usually Universal Time (UT), add 1hr for British Summer Time (BST) ●gives HW & LW times Spring & neap tides ●springs ○higher HW ○lower LW SA SA ●neaps ○lower HW ○higher LW

Chartwork and Position Fixing Tides 6 CPF09 v1.3 Copyright © BSAC 2010 Tide table exercises Exercises ●we need a neap tide to dive a wreck. Which of the three weekends would you recommend and why? 29th – smallest tide ●if we had to dive at LW on 21st July on which tide would you plan the dive? 11:00 – dark at 23:19 ●what will be the states of the tide at 14:00 and at 21:00 on 22nd July? 14:00 flood 21:00 ebb ●what will be the tidal range on the morning of 22nd July? 5.6m ●are the times shown BST or UT? TU 19 W 20 TH 21 F 22 SA SU 24 MO 26 W 27 TH 28 TH 29 SA SU Time m TU 17 MO July

Chartwork and Position Fixing Tides 7 CPF09 v1.3 Copyright © BSAC 2010 Calculating depth 29.5m (CD+HW) 1405 HW 5.5m Charted depths ●depth marked ●lowest predictable level ●does not include the water above chart datum - tides 0800 LW HW LW 1.5m 25.5m (CD+LW) 24m CD 5.5m 1.5m

Chartwork and Position Fixing Tides 8 CPF09 v1.3 Copyright © BSAC 2010 Rule of Twelfths Tide Tables SA Tidal Range 4.8m Divide tidal range by 12 = 0.4m 1 / Planned dive time : = 28.1m 24.9m 2 / m / 12 2 / 12 1 / 12 HW 29.3m LW 24.5m CD 24m Planned dive ●24m CD ●time : m 28.1m 28.9m

Chartwork and Position Fixing Tides 9 CPF09 v1.3 Copyright © BSAC 2010 Depth at any time Actual depth = height of tide above CD ●plus charted depth ●minus drying height ●drying height > height of tide = above water CD+HW-fall of tide CD+LW CD CD+LW+rise of tideheight of tide

Chartwork and Position Fixing Tides 10 CPF09 v1.3 Copyright © BSAC 2010 Depth calculation exercises ●calculate the time and actual depth at afternoon HW on 20 th July. Depth at CD is 25m. ●calculate the time and actual depth at afternoon LW on 28 th July at the same place. ●calculate the actual depth at 1500 on 16 th July. Depth at CD is 0.9m. ●calculate the actual depth at 1830 on 26 th July at a point which dries 1.7m above CD ●Find the standard port for your local area. Enter it and the times and heights of HW and LW on the day of your CPF practical on the voyage plan TU 19 W 20 TH 21 F 22 SA SU 24 MO 26 W 27 TH 28 TH 29 SA SU Time m TU 17 MO July m m 4.2m 2.3m

Chartwork and Position Fixing Tides 11 CPF09 v1.3 Copyright © BSAC 2010 Tidal flow Time of least water movement ●slack water ●longer slack at neaps Slack water Tidal diamonds Table on chart ●hours before and after HW ●direction of flow ●speed on spring tides ●speed on neap tides A

Chartwork and Position Fixing Tides 12 CPF09 v1.3 Copyright © BSAC 2010 Tidal stream atlas Set of diagrams showing currents in area ●Admiralty publications, and in almanacs ●12 or 13 diagrams cover tidal cycle ●time relative to HW Standard Port ●arrows indicate strength & direction ●numbers show speed in 1/10kn ●2.5kn neaps, 5.0kn springs 25,50

Chartwork and Position Fixing Tides 13 CPF09 v1.3 Copyright © BSAC 2010 Tidal stream exercises [Tailor these questions to the local area or replace with similar ones. Supply answers.] 1.what is the rate and direction of the maximum tide at {position} on {date}? 2.at what clock time does this occur? 3.when does slack water occur at {position} and how long does it last? 4.in what direction does the tide at {position} run before LW slack?

Chartwork and Position Fixing Tides 14 CPF09 v1.3 Copyright © BSAC 2010 Tides and currents symbols 3 kn Flood tide stream (with rate) 3 kn Ebb tide stream (with rate) Eddies Overfalls, tide rips, races

Chartwork and Position Fixing Tides 15 CPF09 v1.3 Copyright © BSAC 2010 Implications of tidal flows Slack water diving ●speed of tidal flow < 0.5kn ●not always at HW or LW ●tidal diamonds and atlases fairly reliable offshore ●localised effects around islands, headlands and channels Drift and current-free diving Navigation ●effect of tide on speed and direction ●hazards – turbulent water ●wind over tide Enter details of tidal hazards on the voyage plan

Chartwork and Position Fixing Tides 16 CPF09 v1.3 Copyright © BSAC 2010 Summary Tidal terms Depth and tides Tide tables ●times and heights of HW/LW ●springs and neaps ●depth of water for launch, recovery, diving and navigation Tidal diamonds & atlases ●slack time and duration ●rates for navigation Tidal hazards